Methods and systems for detecting sensor fault modes

ABSTRACT

A method of detecting a fault mode of a sensor is provided. The sensor may be, for example, a bending moment sensor and may sense a bending moment of a blade on a wind turbine generator (WTG). The method includes comparing data output by a first sensor with reference data indicating what is expected to be output by the first sensor to produce a first comparison result and comparing data output by the first sensor with data output by a second sensor to produce a second comparison result. A determination of whether the first sensor has entered a fault mode is made based at least in part on the first and second comparison results.

FIELD OF THE INVENTION

The present invention relates generally to methods and systems fordetecting fault modes in a sensor, and in particular, to methods andsystems for detecting fault modes in a fiber optic sensor used tomeasure a bending moment experienced by a blade of a wind turbinegenerator (WTG).

BACKGROUND OF THE INVENTION

Many modern wind turbine generators (WTGs) are equipped with generallylong and slender blades that are designed to flex or bend in response towind forces. While some bending is expected, the blade can suffer damageif it bends beyond an extreme limit or too frequently. The risk of suchdamage is greater in proportion to the ever increasing size of bladesused in modern WTGs. Therefore, to detect the occurrence of bending,among other reasons, modern WTG blades are sometimes equipped withbending moment measuring sensors that measure a bending momentexperienced at one or more points along the span of each blade. To avoidelectromagnetic interference the sensors are often implemented usingfiber bragg gratings (FBGs) in a fiber optic cable.

Bending moment sensors are not limited to use with blades of a WTG.Therefore, the use of bending moment sensors with WTG blades is merelyone example use. Another example use includes measurement of a bendingmoment in a conduit, such as a marine riser used in deep water oilexploration, or any other structure having a generally long slendershape used in a manner and/or environment that exposes the structure tofrequently changing loads and/or extreme loads. One problem associatedwith the use of any sensor is the risk of the sensor entering a faultmode. A fault mode is a mode in which the sensor behaves in a way thatdeviates from its nominal behavior. The nominal behavior is the one thatfulfills all the requirements imposed by systems that directly orindirectly make use of the sensor's output. Thus, risk of failure orpoor performance for any such systems will increase if the sensor entersa fault mode. Accordingly, identification of the occurrence of a faultmode can help reduce risk of failure and improve performance. Moreover,reducing risk of failure and poor performance is useful for modern WTGsbecause they operate in remote locations, making technician servicingdifficult and costly, and are often required to have long operationallifetimes, e.g., twenty years.

U.S. Patent Application Publication Number 2010/0232961 (the '961 patentpublication) describes one fiber optic sensor that is used to measure abending moment experienced by blade of a WTG. The optical fiber used inthe '961 patent publication has two alternative output points, eachconnectable to a data processing device. Consequently, in the event of abreakage in the optical fiber, signals from the sensor are availablefrom at least one of the output points.

The '961 patent publication appears to address the occurrence of onlyone fault mode, namely, optical fiber breakage. Other sensor fault modesmay occur, however, such as partial or complete loss of power in thesensor output, loss of sensor calibration, and/or signal processingerrors. Moreover, fault modes of a sensor may manifest at a high level(i.e., at a level that takes into account external data or data otherthan that available from the sensor) or at a low level (i.e., at a levelthat takes into account internal data or data available from orpertaining to only the sensor). Identifying other fault modes would helpincrease reliable performance and reduce the risk of failure andassociated repair costs. Furthermore, the ability to distinguish amongdifferent fault modes and fault modes at different levels would improvethe ability to identify appropriate diagnostic techniques to apply.

SUMMARY OF THE INVENTION

According to a first aspect the invention provides a method of detectinga fault mode of a sensor. The sensor may be, for example, a bendingmoment sensor and may sense a bending moment of a blade on a windturbine generator (WTG). The method includes comparing data output by afirst sensor with reference data indicating what is expected to beoutput by the first sensor to produce a first comparison result andcomparing data output by the first sensor with data output by a secondsensor to produce a second comparison result. A determination of whetherthe first sensor has entered a fault mode is made based at least in parton the first and second comparison results. By detecting a fault mode ofthe sensor the risk of undue reliance on the sensor may be reduced in atimely manner without the need for technician servicing.

In an embodiment of the method according to the first aspect of theinvention, the first sensor is configured for use in measuring a firstcharacteristic of a structure and the second sensor is configured foruse in measuring a second characteristic of the structure. In a furtherembodiment, the structure is a wind turbine generator (WTG), the firstcharacteristic of the structure is a bending moment experienced by afirst blade of the WTG, and the second characteristic of the structureis one of: a bending moment experienced by a second blade of the WTG, arotation speed of a generator of the WTG, an acceleration of a nacelleof the WTG, an acceleration of the blade of the WTG, a thrust force ofwind on the WTG, a rotation speed of a rotor of the WTG, and a pitchangle of at least one of the blades of the WTG.

In another embodiment of the method according to the first aspect of theinvention, the first sensor is configured for use in measuring acharacteristic of a structure and the second sensor is configured foruse in measuring a characteristic of the environment in which thestructure is located. In a further embodiment, the structure is a WTG,the characteristic of the structure is a bending moment experienced by ablade of the WTG, and the characteristic of the environment is one of: awind speed, a wind direction, a density of air surrounding the WTG, anda temperature of air surrounding the WTG.

In yet another embodiment of the method according to the first aspect ofthe invention, the method further includes sending an indication of thedetected fault mode to a controller that performs control operations independence on the data output by the first sensor.

In yet another embodiment of the method according to the first aspect ofthe invention, the method further includes sending an indication of thedetected fault mode to a controller of the first sensor, andcompensating, at the sensor controller, for the detected fault mode.

According to a second aspect the invention provides a method fordetecting fault modes of a bending moment sensor for a blade of a WTG.The method includes operating the sensor to generate data representing abending moment experienced by the blade, and comparing the datagenerated by the sensor with reference data indicating what is expectedto be generated by the sensor to produce a comparison result. Adetermination is made as to whether the sensor has entered a fault modebased at least in part on the comparison result.

In one embodiment according to the second aspect of the invention, thesensor includes a fiber bragg grating (FBG) configured for placement ona surface of the blade and sensor circuitry configured to opticallycouple to the FBG via an optical fiber. Moreover, the sensor circuitryincludes a light source configured to emit a light signal and a lightreceiver configured to receive a reflection of the light signal from theFBG and to convert the reflected light signal into the sensor generateddata. Comparing the sensor generated data with the reference data toproduce a comparison result may include comparing a signal power levelindicated by the sensor generated data with a signal power levelindicated by the reference data to produce a power comparison result. Ina further embodiment, the method further includes modifying power outputby the light source in response to determining that the sensor hasentered a fault mode based at least in part on the power comparisonresult.

In an alternative embodiment according to the second aspect of theinvention, the sensor includes the FBG and the sensor circuitrydescribed above. However, in this alternative embodiment, comparing thesensor generated data with the reference data includes comparing a meansignal frequency indicated by the sensor generated data with a meansignal frequency indicated by the reference data to produce a meanfrequency comparison result. In a further embodiment, the method furtherincludes compensating for a frequency offset of the sensor in responseto determining that the sensor has entered a fault mode based at leastin part on the mean frequency comparison result.

According to a third aspect the invention provides a method fordetecting fault modes in one or more of a plurality of sensors. Themethod includes operating first and second sensors, at least the firstsensor producing data representing a bending moment experienced by ablade of the WTG, and comparing data output by the first sensor withdata output by the second sensor to produce a comparison result. Adetermination is then made as to whether the first sensor has entered afault mode based at least in part on the comparison result.

In one embodiment according to the third aspect of the invention, thefirst sensor includes an FBG configured for placement on a surface ofthe blade and sensor circuitry configured to optically couple to the FBGvia an optical fiber. The sensor circuitry includes a light sourceconfigured to emit a light signal, and a light receiver configured toreceive a reflection of the light signal from the FBG and to convert thereflected light signal into the data representing the bending momentexperienced by the blade.

According to a fourth aspect the invention provides an apparatus fordetecting fault modes in a first sensor. The apparatus comprises lowlevel fault detection circuitry and high level fault detectioncircuitry. The low level fault detection circuitry is configured tocompare data output by the first sensor with reference data produced bythe first sensor while operating the first sensor in a controlledenvironment. A fault mode of the first sensor may be detected based atleast in part on the comparison. Moreover, the high level faultdetection circuitry is configured to compare data output by the firstsensor with data output by a second sensor to detect a fault mode of thefirst sensor.

According to a fifth aspect the invention provides a WTG with low levelsensor fault detection. The WTG includes a blade configured to rotate inresponse to a wind force and a sensor configured for use in measuring abending moment experienced by the blade. The WTG further includes faultdetection circuitry that is configured to compare data output by thesensor during operation with the reference data to determine whether thesensor has entered a fault mode.

According to a sixth aspect the invention provides a WTG with high levelsensor fault detection. The WTG includes a first blade configured torotate in response to a wind force and a first sensor configured for usein measuring a bending moment experienced by the first blade. The WTGfurther includes fault detection circuitry that is configured to comparedata output by the first sensor with data output by a second sensor todetermine whether the first sensor has entered a fault mode.

In one embodiment according to the sixth aspect of the invention, theWTG further comprises a second blade and the second sensor is configuredfor use in measuring a bending moment experienced by the second blade.

In another embodiment according to the sixth aspect of the invention,the second sensor is configured for use in measuring a characteristic ofthe WTG other than a bending moment experienced by a blade.

In yet another embodiment according to the sixth aspect of theinvention, the second sensor is configured for use in measuring acharacteristic of the environment in which the WTG is located.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood with reference to the detaileddescription when considered in conjunction with the non-limitingexamples and the accompanying drawings.

FIG. 1 shows a general structure of a WTG, which is an example structurethat uses sensors for which a fault mode may be detected.

FIG. 2 shows a system for detecting a fault mode for a load sensor on aWTG, such as the WTG of FIG. 1.

FIG. 3 shows a more detailed view of certain portions of the system inFIG. 2.

FIG. 4 shows a graph of on which is depicted both an example nominalsignal and an example faulty signal that may be produced by the loadsensor in FIGS. 2 and 3.

FIG. 5 illustrates a flow diagram representing an example method ofdetecting whether a sensor has entered a fault mode.

DETAILED DESCRIPTION OF THE INVENTION

The following is a detailed description of embodiments of the inventiondepicted in the accompanying drawings. The embodiments are examples andare in such detail as to clearly communicate the invention. However, theamount of detail offered is not intended to limit the anticipatedvariations of embodiments; but on the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the present invention as defined by the appendedclaims.

Furthermore, in various embodiments the invention provides numerousadvantages over the prior art. However, although embodiments of theinvention may achieve advantages over other possible solutions and/orover the prior art, whether or not a particular advantage is achieved bya given embodiment is not limiting of the invention. Thus, the followingaspects, features, embodiments and advantages are merely illustrativeand are not considered elements or limitations of the appended claimsexcept where explicitly recited in a claim(s). Likewise, reference to“the invention” shall not be construed as a generalization of anyinventive subject matter disclosed herein and shall not be considered tobe an element or limitation of the appended claims except whereexplicitly recited in a claim(s).

Wind turbine generators (WTGs) are subject to loading due to windforces, rotational motion, gravity, and the like. This loading isusually undesirable and is therefore often monitored and sometimescontrolled with the use of sensors. For example, the blades of somemodern wind turbines have stress sensors or gauges mounted typically onan inner surface thereof. The stress sensors are capable of measuringand reporting to a central controller an amount of stress experienced bythe blade. However, WTGs are often in remote locations and are designedto operate with little maintenance and supervision. Therefore,occurrence of a fault mode in a sensor may escape detection. If controlof the WTG depends on a sensor that experiences a fault mode, the WTGmay operate inefficiently, or worse, may suffer damage. Example methodsand systems described herein can be used to detect fault modes of asensor in a WTG, thereby facilitating avoidance of such risks. Moreover,although a WTG is frequently referred to herein as an example structurethat uses sensors, other structures are known to include similar sensorsthat can enter fault modes. Therefore, the methods and systems describedherein are not limited to use with WTGs.

FIG. 1 illustrates an example WTG 100 according to an embodiment. Asillustrated in FIG. 1, the WTG 100 includes a tower 110, a nacelle 120,and a rotor 130. In one embodiment, the WTG 100 may be an onshore windturbine. However, embodiments of the invention are not limited only toonshore wind turbine. In alternative embodiments, the wind turbine 100may be an offshore wind turbine located over a water body such as, forexample, a lake, an ocean, or the like. The tower 110 of such anoffshore wind turbine is installed on either the sea floor or onplatforms stabilized on or above the sea level.

The tower 110 of the WTG 100 may be configured to raise the nacelle 120and the rotor 130 to a height where strong, less turbulent, andgenerally unobstructed flow of air may be received by the rotor 130. Theheight of the tower 110 may be any reasonable height, and shouldconsider the length of wind turbine blades extending from the rotor 130.The tower 110 may be made from any type of material, for example, steel,concrete, or the like. In some embodiments the tower 110 may be madefrom a monolithic material. However, in alternative embodiments, thetower 110 may include a plurality of sections. In some embodiments ofthe invention, the tower 110 may be a lattice tower. Accordingly, thetower 110 may include welded steel profiles.

The rotor 130 may include a rotor hub (hereinafter referred to simply asthe “hub”) 132 and at least one blade 140 (three such blades 140 areshown in FIG. 1). The rotor hub 132 may be configured to couple the atleast one blade 140 to a shaft (not shown). In one embodiment, theblades 140 may have an aerodynamic profile such that, at predefined windspeeds, the blades 140 experience lift, thereby causing the blades toradially rotate around the hub. The hub 132 may further comprisemechanisms (not shown) for adjusting the pitch of the blade 140 toincrease or reduce the amount of wind energy captured by the blade 140.Pitching adjusts the angle at which the wind strikes the blade 140. Incertain embodiments, however, the pitching mechanisms may be omittedand, consequently, the pitch of the blades 140 cannot be adjusted insuch embodiments.

The hub 132 typically rotates about a substantially horizontal axisalong a drive shaft (not shown) extending from the hub 132 to thenacelle 120. The drive shaft is usually coupled to one or morecomponents in the nacelle 120, which are configured to convert therotational energy of the shaft into electrical energy.

Although the WTG 100 shown in FIG. 1 has three blades 140, it should benoted that a WTG may have a different number of blades. It is common tofind WTGs having two to four blades. The WTG 100 shown in FIG. 1 is aHorizontal Axis Wind Turbine (HAWT) as the rotor 130 rotates about ahorizontal axis. It should be noted that the rotor 130 may rotate abouta vertical axis. Such a WTG having its rotor rotate about the verticalaxis is known as a Vertical Axis Wind Turbine (VAWT). The WTGembodiments described henceforth are not limited to HAWTs having 3blades. They may be implemented as both HAWTs and VAWTs, having anynumber of blades 140 in the rotor 130.

Each of the blades 140 may also be equipped with a bending moment sensor142 (i.e., a load sensor), such as a strain-gauge, accelerometer,vibration sensor, or any other type of sensor capable of measuring atleast a magnitude of a bending moment experienced by a WTG blade. Thebending moment sensor 142 may be positioned at a root end of the bladeto sense stress due to a flap bending moment of the blade, i.e., amoment that causes the blade to deflect in a direction normal to theplane of the rotor 130. Although the wind turbine 100 is depicted ashaving only one bending moment sensor 142 on each blade 140, multiplebending moment sensors 142 may be included on each blade (or fewer thaneach blade) at various positions, e.g., at 20%, 40%, 50%, 60%, 75% and80% of the blade radius from the blade root. Moreover, at least some ofthe multiple bending moment sensors 142 (or, alternatively, at least oneadditional bending moment sensor) may be positioned to measure an edgebending moment, i.e., a moment that causes the blade to deflect in adirection substantially within the plane of the rotor 130, rather than aflap bending moment. Alternatively, only a single one of the blades 140may be equipped with one or more bending moment sensors 142.

Furthermore, in addition to positioning one or more bending momentsensors 142 on or within each blade 140, one or more additional sensorsmay be used to control or log data about operation of the WTG 100 or itsoperating environment. For example, the WTG 100 may include a sensor atthe back of the nacelle 120 in the form of an accelerometer. Theaccelerometer may be mounted in such a way that the accelerometermeasures horizontal or substantially horizontal oscillations of thenacelle, which may result from edgewise oscillations of the blades.Other possible sensors include a wind speed sensor, a wind directionsensor, a wind thrust force sensor, a generator speed sensor, atemperature sensor, an air density sensor, a WTG location detectingsensor (e.g. a GPS receiver), a blade pitch angle sensor, a blade angleof attack sensor, a tower top acceleration sensor, a rotor speed sensor,etc. Furthermore, some sensors included on the WTG may be configured foruse in measuring certain characteristics of the WTG (including, e.g.,characteristics that the foregoing sensors measure more directly) basedon a physical model and/or knowledge of operating parameters, such asWTG altitude, blade masses and inertia, a blade length, and/or a gearboxratio. Sensors other than the bending moment sensors 142 are sometimesreferred to in the description and figures as “other sensors” forbrevity.

FIG. 2 shows a functional block diagram of an example system 200 thatimplements an example method of detecting fault modes of one or moresensors. FIG. 3 shows a functional block diagram of certain componentsin the system 200 in greater detail. Thus, the following description ofthe system 200 is made with reference to both FIGS. 2 and 3.

In the example system shown, the one or more sensors are blade bendingmoment sensors 142 (i.e., load sensors) on blades of the WTG 100.However, the system 200 is not limited to use with load sensors onblades of a WTG and may instead be used in other contexts and with othertypes of sensors. Moreover, it should be noted that the number of bladesn shown in FIG. 2 may be any positive integer. In addition, the numberof load sensors m may be any positive integer and may differ from thenumber of blades n. For example, fewer than all blades may be equippedwith load sensors. Alternatively, each blade may be equipped with a loadsensor. In addition, or alternatively, one or more additional componentsof the WTG (e.g., the tower) may be equipped with a corresponding loadsensor.

The system 200 also includes low level fault detectors 220 for each loadsensor and a high level fault detector 240. The load sensors 142, lowlevel fault detectors 220, and high level fault detector 240 produceoutputs that are fed to a WTG controller 230 that in turn produces oneor more outputs that are fed to one or more WTG actuators or othercontrollable components. For example, the load sensors 142 generatebending moment data (also referred to herein as strain data) that is fedto the WTG controller 230 and the WTG controller 230 sends commands toactuators, such as pitch actuators that control blade pitch, independence on the bending moment data to control the bending momentexperienced by the blades. The pitch actuators may be individually orcollectively controllable. Other actuators that may be controlled independence on bending moment data include yaw motors and components thataffect generator torque, speed, and/or power.

Each load sensor 142 is subject to entering fault modes duringoperation. The low level fault detectors 220 and high level faultdetector 240 are configured to detect at least some of the fault modes,the high level fault detector 240 detecting fault modes that might bemissed by the low level fault detectors 220 (and vice-versa). Each lowlevel fault detector 220 is associated with a corresponding load sensor142. Each low level fault detector 220 compares data output by thecorresponding one of the load sensors 142 with reference data expectedto be output by the load sensor 142. Each low level fault detector 220performs the comparison and produces a comparison result (i.e. a lowlevel validity indication) indicating whether the output data matchesthe reference data. The comparison may be implemented using astatistical test, such as a generalized likelihood ratio test or acumulative sum test. The statistical test may be designed to detectsmall changes with a designed probability of missed detection and/orfalse alarm. Accordingly, the reference data may be provided in the formof a mean value and, optionally, may include a variance value derivedfrom a statistically significant number of output samples.

The low level validity indication is fed to the WTG controller 230.Based at least in part on the low level validity indication, the WTGcontroller 230 determines whether the load sensor 142 has entered afault mode and may optionally take measures to address the fault modeby, e.g., controlling the faulty load sensor to compensate for the faultmode and/or by reducing or eliminating the dependence of any of itsactuator control operations on the output of the faulty load sensor. Thelow level validity indication may be produced in the form of a truthtable listing a binary value (e.g., valid/not valid) for each of aplurality of possible fault modes, or may be in the form of multi-levelor graded values indicating a degree of to which the load sensor isoperating in a fault mode for each possible fault mode. Alternatively,the low level validity indication may be some combination of a truthtable and graded values. Example fault modes are discussed below withreference to FIGS. 3 and 4.

The reference data may be derived using a model derived from knowledgeof the properties of the sensor. For example, the reference data can beobtained based on design and/or specification parameters provided indocumentation produced by a manufacturer of the sensor and/or fromobserved conditions in which the sensor is operating. Alternatively, thereference data may be produced by each sensor during an initializationphase in which each load sensor 142 is operated in a controlledenvironment. The controlled environment may be one in which the blade isexperiencing substantially no loading or bending moment. Such anenvironment may be achieved, for example, by pitching the blades acertain direction, waiting until little or no wind is present, or in amanufacturing facility during an assembly phase of the WTG.Alternatively, if the performance of each load sensor 142 has little tono variation with respect to the performance of other load sensors 142,the reference data for all low level fault detectors 220 may be providedby operation of a single one of the load sensors 142 or by operation ofa reference load sensor that is not provided on the WTG 100.

Moreover, each low level fault detector 220 may include therein a memorymodule that stores the reference data. Alternatively, the system 200 mayinclude a single memory module that stores the reference data for eachlow level fault detector 220 to reference and use during its comparisonoperation. Moreover, although multiple load sensors are shown in thesystem 200, embodiments having only a single load sensor associated witha single blade of a multi-blade WTG are also contemplated.

As shown in FIG. 3, the load sensors 142 may be implemented using sensorcircuitry 310 and an optical fiber 320 having one or more phase masks,which may be in the form of fiber bragg gratings (FBG) 330. To avoidundue complexity, FIG. 2 depicts the entirety of each of load sensors142 located inside a hollow center portion of a blade of the WTG 100.However, as shown in FIG. 3, only the FBGs 330 of each load sensor 142and a portion of the optical fiber 320 may actually be located insidethe corresponding blade. The sensor circuitry 310 may reside in acentral hub or elsewhere in the WTG 100 and may be communicativelycoupled to the FBGs 330 via a portion of the optical fiber 320. In oneembodiment the load sensors 142 are implemented with FBGs 330 andsensory circuitry 310 that are manufactured by Ibsen Photonics, acompany with headquarters located at Ryttermarken 15-21, DK-3520 Farum,Denmark. For example, Ibsen Photonics' I-MON 80D interrogation monitormay be used.

The sensor circuitry 310 includes an interrogation monitor 311, which inturn includes a light source 312 (e.g., a superluminescent diode (SLED)or an amplified spontaneous emission (ASE) source), an opticalcirculator 314, a spectrum monitor 316, and control circuitry 318 thatcontrols operation of the light source 312 and the spectrum monitor 316.In addition, the sensor circuitry 310 includes a converter 319 thatconverts an output of the interrogation monitor 311 into bending momentdata that is sent to the WTG controller 230. Although the optical fiber320 is shown as providing a link between the interrogation monitor 311and the FBGs 330 of a single load sensor in a single blade, theinterrogation monitor 311 may be linked by the optical fiber 320 tomultiple, separately located load sensors.

In operation, the light source 312 emits broadband light into theoptical fiber 320. Each FBG 330 reflects a peak or narrow band of thebroadband light. Moreover, each FBG's reflected peak may be centeredaround a characteristic wavelength (depicted in FIG. 3 as λ1, λ2, . . .λn) that differs from the characteristic wavelengths of the other FBGs330 in the optical fiber 320 to facilitate identification of whichreflection corresponds to which FBG. The FBGs 330 are mounted on aninner surface of the blade, which stretches and compresses duringoperation. Therefore, the FBGs 330 also undergo the stretching andcompressing along with the blade, which affects the frequency responseof the FBGs. Specifically, the FBG's characteristic wavelength willshift in proportion to the bending or compression experienced, therebyproviding a measure of the bending moment experienced by the blade.

The interpretation of characteristic wavelength shifts to bending momentdata is accomplished by the spectrum monitor 316, control circuitry 318,and converter 319. More specifically, the peaks of reflected light arereflected back toward the interrogation monitor 310. The opticalcirculator 314 directs the reflected light to the spectrum monitor 316and the spectrum monitor includes a light receiver that converts thereceived optical reflections into electrical signals that carry bendingmoment data and that are fed to the control circuitry 318. The controlcircuitry 318 detects the frequencies of the different peaks ofreflected light and determines an amount by which the frequencies areshifted. This information is fed to the converter 319 for conversion tobending moment data using a predetermined mathematical relationship.However, if the load sensor 142 enters a fault mode, the bending momentdata can become unreliable, which can degrade control operations thatdepend on the bending moment data, thereby degrading performance of theWTG 100 and potentially causing harmful damage.

FIG. 4 shows a power versus frequency graph on which is depicted anominal optical signal 410 reflected by a particular one of the FBGs 330that is not experiencing a bending moment. In accordance with thediscussion above regarding the FBGs 330, the nominal optical signal 410has a peak that is centered around a characteristic wavelength that isunique to the associated FBG 330. A fault mode of the associated loadsensor 142 may be exhibited by a change of power and/or change ofcharacteristic wavelength of the nominal optical signal 410. Forexample, the FBG 330 may enter a fault mode in which it produces afaulty optical signal 420 that is lower in power and/or higher infrequency than the nominal optical signal 410. Such fault modes mayoccur due to, for example, temperature swings, hardware and/or softwarebugs, short circuits, general wear and tear, etc. Furthermore, althoughthe faulty optical signal 420 suffers changes in both power andfrequency, the faulty optical signal 420 may instead suffer only achange in power or only a change in frequency. Moreover, a fault modemay be exhibited in other ways including, for example, power increase, adownshift in frequency, excess signal noise, etc., in the faulty opticalsignal 420.

During normal operation, the spectrum monitor 316 outputs the nominaloptical signal 410 (and may output other similar optical signalscorresponding to other FBGs 330, if present). This output data is fednot only to the control circuitry 318, but also to the low level faultdetector 220, which compares the output data with reference dataexpected to be output by the spectrum monitor 316. Comparing the outputdata with reference data may include comparing a signal power levelindicated by the output data with a signal power level indicated by thereference data to produce a power comparison result (e.g., a valuerepresenting the Δ_(power) quantity shown in FIG. 4). In addition, oralternatively, the comparing the output data with reference data mayinclude comparing a mean signal frequency indicated by the output datawith a mean signal frequency indicated by the reference data to producea mean frequency comparison result (e.g., a value representing theΔ_(freq) quantity shown in FIG. 4). Another check that may be performedby low level fault detector 220 is a peak count to make sure an expectednumber of peaks are received. For example, if five FBGs 330 are includedin the load sensor under test but only four peaks are counted thisresult will indicate a fault mode. The peak count, power comparisonresult and/or mean frequency comparison result may be provided as partof the low level validity indication.

By performing the foregoing comparison(s), the low level fault detector220 is able to detect if the load sensor 142 has entered a fault mode.For example, during faulty operation, the spectrum monitor 316 mightoutput the faulty optical signal 420, which, as discussed above, wouldbe detected as a faulty signal by the statistical comparison operationsperformed by the low level fault detector 220. The low level faultdetector 220 may then output a negative low level validity indication tothe WTG controller 230 as a warning for the WTG controller 230 to ignoreor reduce dependence on the faulty load sensor and/or to take correctiveaction. In addition, or alternatively, the negative low level validityindication may be sent to the control circuitry 318 as a warning for thecontrol circuitry 318 to ignore the sensor output and/or to takecorrective action. Moreover, the negative low level validity indicationmay include information about which particular FBG or FBGs are faulty ifonly one or a select number of FBGs are found to be faulty.

Some fault modes may not be detected by the low level fault detector 220or may not be detected in a timely manner. Therefore, the system 200 mayalso include a high level fault detector 240. The high level faultdetector 240 may be fed the same output data as each low level faultdetector 240 but may compare the output data to other data to produce asecond comparison result (i.e., a high level validity indication). Thehigh level validity indication may be provided in the form of a truthtable, graded values, or some combination thereof, as discussed abovewith respect to the low level validity indication. Moreover, the otherdata evaluated by the high level fault detector 240 may be received from(or derived from data received from) other sensors, including sensorsthat are configured for use in measuring characteristics of the WTGother than a blade bending moment, such as a generator speedmeasurement, a generator power measurement, a generator torquemeasurement, a nacelle acceleration measurement, a tower accelerationmeasurement, a blade acceleration measurement, a WTG locationmeasurement, a blade pitch angle measurement, a blade pitch angle ofattack measurement, a rotor speed measurement, and/or an internaltemperature measurement. In addition, or alternatively, the other datamay be received from (or derived from data received from) sensors thatare configured for use in measuring characteristics of the environmentin which the WTG operates, such as a wind speed measurement, a winddirection measurement, a wind thrust force measurement, an air densitymeasurement, and/or an ambient temperature measurement. Receiving datafrom other sensors facilitates detection and diagnosis of fault modes inthe load sensors 142 that may not be easily detected by the low levelfault detector 220, including long term frequency drifts of an FBG 330.In addition, or alternatively, the high level fault detector 240 maycompare the data output from a first one of the load sensors 142 withthe data output from a second one of the load sensors 142. The highlevel fault detector 240 may also be fed the bending moment or straindata produced by the converter 319 of the load sensor 142 to verifycorrect operation of the converter 319.

The data output from each load sensor 142 may be affected by the azimuthangle of the corresponding blade due to, for example, tower shadoweffects on loads. Therefore, as part of comparing data output from afirst load sensor 142 with data output from a second load sensor 142,the high level fault detector 240 may temporally shift one or both setsof data to align the azimuth angles of each set of data. To facilitatesuch temporal shifting, the high level fault detector 240 may receivemeasurements made by an azimuth angle sensor or may derive the azimuthangle from load sensor data. Such a comparison may not only compensatefor difference in azimuth angle but also for rotor speed and/orturbulence intensity.

Moreover, when comparing data output by different types of sensors thehigh level fault detector 240 may refer to a predetermined relationshipthat relates the two sets of data. For example, the data output by awind speed sensor cannot directly be compared to data output by the loadsensors 142. Thus, one or both sets of data may be processed accordingto a predetermined relationship that may be derived empirically and/orfrom known physical laws that relate the sets of data as part of thecomparison operation. Moreover, the processing may depend on additionaldata from other sensors and/or known parameters. For example, anestimate of expected wind speed may be derived from load sensor datausing a predetermined relationship and the estimated wind speed may becompared to the wind speed measured by a sensor to determine whether theload sensor producing the load sensor data is faulty. Other example highlevel detection operations may be performed using one or more residualgenerators created based on models of WTG operations that at leastpartially depend on or affect the outputs of the load sensors 142according to certain known physical relationships (derived empiricallyand/or from physical laws). Residuals produced by residual generatorsare signals defined to have a zero mean and a known standard deviation.Thus, a fault mode can be detected when the mean of a residual deviatessignificantly from zero or the standard deviation of the residualdeviates significantly from the known standard deviation. The amount ofdeviation can be measured by a statistical test, such as a generalizedlikelihood ratio test or a cumulative sum test, which facilitate tuningthe likelihood of false alarms and/or missed detections.

The low level and high level fault detectors 220, 240 may perform faultdetection automatically at regular or irregular intervals and/or inresponse to a command from a controller, such as the WTG controller 230.Moreover, the low level and high level fault detectors 220, 240 may beimplemented using circuitry that includes hardware, software encoded oncomputer-readable media including programmable and non-programmablemedia, or any combination of the foregoing. When a fault mode isdetected by either the low level or high level fault detector 220, 240,the detector may send an indication of the detected fault mode to theWTG controller 230 or the control circuitry 318. The WTG controller 230or the control circuitry 318 may then compensate for the detected faultmode. Compensation may include, for example, ignoring the output of thefaulty load sensor. Alternatively, the control circuitry 318 may controlthe light source 312 to boost power based on the Δ_(power) valuereported by the low level fault detector 220 and/or may process theoutput of the spectrum monitor 316 based on the Δ_(freq) value reportedby the low level fault detector 220.

The foregoing systems may be used to implement various different faultmode detecting methods. FIG. 5 shows a flow diagram representing anexample method 500 of detecting a fault mode in a sensor, such as one ofthe load sensors 142. The method may be carried out by the system 200 inFIGS. 2 and 3. At stage 510, a first sensor, such as one of the loadsensors 142, is operated to generate a first set of data (i.e., a firstset of one or more data points) representing a bending momentexperienced by a blade of a WTG. At stages 520 and 530, fault modedetection is performed at a low level by comparing the first set of datawith a set of reference data to produce one or more comparison results.More specifically, at stage 520 a signal power level indicated by thefirst set of data is compared with a signal power level indicated by theset of reference data to produce a power comparison result and at stage530 a mean signal frequency indicated by the first set of data iscompared with a mean signal frequency indicated by the set of referencedata to produce a mean frequency comparison result. The low level faultdetector 220 may perform the foregoing comparison stages 520 and 530.

Although both of the comparison stages 520 and 530 are depicted as beingperformed, in an alternative method only a single one of the stages 520and 530 is performed. In addition to or instead of performing one orboth of the comparison stages 520 and 530, additional comparison stagesmay be performed to compare other aspects of the first set of data withcorresponding aspects of the set of reference data.

In addition to performing low level fault mode detection, the method 500includes stages 540 and 550 that pertain to high level fault modedetection. The high level fault mode detection stages may be performedin parallel (as shown) or in series with the foregoing low level faultmode detection stages 520 and 530.

The high level fault mode detection stages include stages 540 and 550.At stage 540, a second sensor is operated to generate a second set ofdata. The second set of data may represent a bending moment experiencedby another blade of the WTG. Alternatively, the second set of data mayrepresent a bending moment experienced by another, non-blade componentof the WTG or may represent other characteristics of the WTG or itsoperating environment, such as wind speed, wind direction, generatorspeed, generator torque, or the like. At stage 550, the first set ofdata is compared with the second set of data to produce a high levelcomparison result. The high level fault detector 240 may perform thiscomparison operation. As part of the comparison, one or both sets ofdata may be processed, if necessary. For example, an estimate of thesecond set of data may be generated based on the first set of data (orvice-a-versa) for meaningful comparison with the second set of data.This data processing may also be performed by the high level faultdetector 240.

At stage 560 a determination is made as to whether the first sensor hasentered a fault mode based at least in part on one or more of thecomparison results. This determination may be made by either the lowlevel fault detector 220 or the high level fault detector 240. It ispossible for both the low level fault detector 220 and the high levelfault detector to make the determination that the first sensor hasentered a fault mode. For example, the low level fault detector 220 maymake a determination of whether the first sensor has entered a faultmode based on the low level comparison results (power and mean frequencycomparison results) and the high level fault detector 240 may make asimilar determination based on the high level comparison result. Thesedeterminations may be based on multiple data set comparisons made atdifferent times at both the low level and high level. Moreover, at thehigh level the determination may be made based on various different setsof data from different sensors of the same type or different types.

At stage 570, an indication of the detected fault mode is sent to acontroller of the first sensor, such as the WTG controller 230 and/orthe control circuitry 318, and compensation is performed at the sensorcontroller for the detected fault mode. The indication of fault mode maybe sent by either the low level fault detector 220 or the high levelfault detector 240. Moreover, if the first sensor is the load sensor 142and the fault mode is detected at the low level, the compensation mayinclude modifying power output by the light source 312 based at least inpart on the power comparison result and/or compensating for a frequencyoffset based at least in part on the mean frequency comparison result.Compensation for the detected fault mode may also include disabling thefirst sensor and/or reducing or eliminating the dependence of anyactuator control operations on the output of the first sensor.

The method 500 is provided by way of example, not limitation, and may bemodified in various ways, including omission and/or repetition ofcertain stages, as well as addition of other stages. For example, in onealternative embodiment of the method 500, the low level fault detectordoes not send an indication of the detected fault to a sensor controllerat stage 570 and instead controls the first sensor to compensate for thefault mode. Moreover, in other alternative embodiments of the method500, either the low level fault mode detection stages (stages 520 and530) or the high level fault mode detection stages (stages 540 and 550)are omitted and only a single level (low or high) of fault modedetection is performed. Furthermore, stage 570 may be omitted in someembodiments if, for example, the sensor controller has no way ofcompensating for the detected fault mode or if such compensation isimpractical. Thus, the determination of whether the first sensor hasentered a fault mode may simply be flagged for a technician to service(e.g., by inspecting, repairing, and/or replacing) the faulty sensorimmediately or during a scheduled maintenance operation. Furthermore,the method 500 may be adapted for use with load sensors used onstructures other than WTGs and/or with types of sensors other than loadsensors.

In addition, the method 500 or portions thereof may be repeated asnecessary. For example, if the high level comparison result indicates afault mode and the low level comparison result is unavailable oruncertain it may not be clear whether the first sensor or the secondsensor is faulty. Therefore, stages 540 and 550 may be repeated one ormore additional times using additional data sets from the same or othersensors (if available) to aid in determining whether the first sensor orsecond sensor is faulty. Moreover, additional stages that are not shownmay be added to the method 500. For example, a stage for counting anumber of peaks received by the load sensor 142 may be included todetect a fault mode in which a number of peaks that are received doesnot correspond to a number of FBGs in the load sensor 142.

Example methods and systems described herein may be used to detect faultmodes of a sensor. The sensor in one example embodiment is a bendingmoment sensor used in a blade of a WTG. The example methods and systemsinclude fault detection at a low level and/or at a high level. At thelow level, the output of the sensor is compared to reference data and atthe high level the output of the sensor is compared to the output ofother sensors, which may include other types of sensors that measureother types of data.

It should be emphasized that the embodiments described above arepossible examples of implementations which are merely set forth for aclear understanding of the principles of the invention. The personskilled in the art may make many variations and modifications to theembodiment(s) described above, said variations and modifications areintended to be included herein within the scope of the following claims.

1. A method for detecting fault modes of a sensor, the method comprising: comparing data output by the first sensor with reference data indicating what is expected to be output by the first sensor to produce a first comparison result; comparing data output by the first sensor with data output by a second sensor to produce a second comparison result; and determining whether the first sensor has entered a fault mode based at least in part on the first and second comparison results.
 2. The method of claim 1, wherein the first sensor is configured for use in measuring a first characteristic of a structure and the second sensor is configured for use in measuring a second characteristic of the structure.
 3. The method of claim 2, wherein the structure is a wind turbine generator (WTG), the first characteristic of the structure is a bending moment experienced by a first blade of the WTG, and the second characteristic of the structure is one of: a bending moment experienced by a second blade of the WTG, a rotation speed of a generator of the WTG, an acceleration of a nacelle of the WTG, an acceleration of the first blade of the WTG, a thrust force of wind on the WTG, a rotation speed of a rotor of the WTG, and a pitch angle of at least one of the blades of the WTG.
 4. The method of claim 1, wherein the first sensor is configured for use in measuring a characteristic of a structure and the second sensor is configured for use in measuring a characteristic of the environment in which the structure is located.
 5. The method of claim 4, wherein the structure is a WTG, the characteristic of the structure is a bending moment experienced by a blade of the WTG, and the characteristic of the environment is one of: a wind speed, a wind direction, a density of air surrounding the WTG, and a temperature of air surrounding the WTG.
 6. The method of claim 1, further comprising: sending an indication of the detected fault mode to a controller that performs control operations in dependence on the data output by the first sensor.
 7. The method of claim 1, further comprising: sending an indication of the detected fault mode to a controller of the first sensor; and compensating, at the sensor controller, for the detected fault mode.
 8. A method for detecting fault modes of a bending moment sensor for a blade of a WTG, the method comprising: operating the sensor to generate data representing a bending moment experienced by a blade of a WTG; and comparing the data generated by the sensor with reference data indicating what is expected to be generated by the sensor to produce a comparison result; and determining whether the sensor has entered a fault mode based at least in part on the comparison result.
 9. The method of claim 8, wherein the sensor includes: a fiber bragg grating (FBG) configured for placement on a surface of the blade; and sensor circuitry configured to optically couple to the FBG via an optical fiber, the sensor circuitry including: a light source configured to emit a light signal; and a light receiver configured to receive a reflection of the light signal from the FBG and to convert the reflected light signal into the sensor generated data, wherein comparing the sensor generated data with the reference data to produce a comparison result includes comparing a signal power level indicated by the sensor generated data with a signal power level indicated by the reference data to produce a power comparison result.
 10. The method of claim 9, wherein the method further includes: modifying power output by the light source in response to determining that the sensor has entered a fault mode based at least in part on the power comparison result.
 11. The method of claim 8, wherein the sensor includes: a fiber bragg grating (FBG) configured for placement on a surface of the blade; and sensor circuitry configured to optically couple to the FBG via an optical fiber, the sensor circuitry including: a light source configured to emit a light signal; and a light receiver configured to receive a reflection of the light signal from the FBG and to convert the reflected light signal into the sensor generated data, wherein comparing the sensor generated data with the reference data to produce a comparison result includes comparing a mean signal frequency indicated by the sensor generated data with a mean signal frequency indicated by the reference data to produce a mean frequency comparison result.
 12. The method of claim 11, wherein the method further includes: compensating for a frequency offset of the sensor in response to determining that the sensor has entered a fault mode based at least in part on the mean frequency comparison result.
 13. A method for detecting fault modes in a sensor, the method comprising: operating first and second sensors, at least the first sensor producing data representing a bending moment experienced by a blade of a WTG; comparing data output by the first sensor with data output by the second sensor to produce a comparison result; and determining whether the first sensor has entered a fault mode based at least in part on the comparison result.
 14. The method of claim 13, wherein the first sensor includes: a fiber bragg grating (FBG) configured for placement on a surface of the blade; sensor circuitry configured to optically couple to the FBG via an optical fiber, the sensor circuitry including: a light source configured to emit a light signal; and a light receiver configured to receive a reflection of the light signal from the FBG and to convert the reflected light signal into the data representing the bending moment experienced by the blade.
 15. An apparatus for detecting fault modes in a first sensor, the apparatus comprising: low level fault detection circuitry configured to compare data output by the first sensor with reference data to detect a fault mode of the first sensor; and high level fault detection circuitry configured to compare data output by the first sensor with data output by a second sensor to detect a fault mode of the first sensor.
 16. A wind turbine generator (WTG) with low level sensor fault detection, the WTG comprising: a blade configured to rotate in response to a wind force; a sensor configured for use in measuring a bending moment experienced by the blade; and fault detection circuitry configured to compare data output by the sensor during operation with reference data to determine whether the sensor has entered a fault mode.
 17. A WTG with high level sensor fault detection, the WTG comprising: a first blade configured to rotate in response to a wind force; a first sensor configured for use in measuring a bending moment experienced by the first blade; fault detection circuitry configured to compare data output by the first sensor with data output by a second sensor to determine whether the first sensor has entered a fault mode.
 18. The WTG of claim 17, wherein the WTG further comprises a second blade and the second sensor is configured for use in measuring a bending moment experienced by the second blade.
 19. The WTG of claim 17, wherein the second sensor is configured for use in measuring a characteristic of the WTG other than a bending moment experienced by a blade of the WTG.
 20. The WTG of claim 17, wherein the second sensor is configured for use in measuring a characteristic of the environment in which the WTG is located. 